Technological development and increased demand for mobile equipment have led to a sharp rise in the demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and voltage, long cycle lifespan and low self-discharge are commercially available and widely used.
In addition, increased interest in environmental issues has brought about a great deal of research associated with electric vehicles (EVs), hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles as alternatives to vehicles using fossil fuels such as gasoline vehicles and diesel vehicles which are major causes of air pollution. Nickel-metal hydride (Ni-MH) secondary batteries are generally used as power sources of electric vehicles (EVs) and hybrid electric vehicles (HEVs). However, a great deal of study associated with use of lithium secondary batteries with high energy density, discharge voltage and power stability is currently underway and some are commercially available.
A lithium secondary battery has a structure in which a non-aqueous electrolyte comprising a lithium salt is impregnated into an electrode assembly comprising a cathode and an anode, each comprising an active material coated on a current collector, and a porous separator interposed therebetween.
Lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium composite oxide and the like are generally used as cathode active materials of lithium secondary batteries and carbon-based materials are generally used as anode active materials thereof. Use of silicon compounds, sulfur compounds and the like has also been considered.
However, lithium secondary batteries have various problems, in particular, problems associated with fabrication and driving properties of an anode.
First, regarding fabrication of an anode, a carbon-based material generally used as an anode active material is highly hydrophobic and thus has problems of low miscibility with a hydrophilic solvent and low dispersion uniformity of solid components in the process of preparing a slurry for electrode fabrication. In addition, this hydrophobicity of the anode active material complicates impregnation of highly polar electrolytes in the battery fabrication process. The electrolyte impregnation process is a bottleneck in the battery fabrication process, thus greatly decreasing productivity.
In order to solve these problems, addition of a surfactant to an anode, an electrolyte or the like is suggested. However, disadvantageously, the surfactant may have side effects on driving properties of batteries.
Meanwhile, regarding driving properties of the anode, disadvantageously, the carbon-based anode active material induces initial irreversible reaction, since a solid electrolyte interface (SEI) layer is formed on the surface of the carbon-based anode active material during an initial charge/discharge process (activation process), and battery capacity is reduced due to exhaustion of the electrolyte caused by removal (breakage) and regeneration of the SEI layer during a continuous charge/discharge process.
In order to solve these problems, various methods such as formation of an SEI layer through stronger bond or formation of an oxide layer on the surface of the anode active material have been attempted. These methods have properties unsuitable for commercialization such as deterioration in electrical conductivity caused by the oxide layer and deterioration in productivity caused by additional processes.
Furthermore, it is not easy to form oxide layers having different physical properties on a non-polar anode active material and thus formation of a uniform oxide layer inevitably increases process cost.
Accordingly, there is an increasing need for methods capable of ultimately solving these problems.